Scanning probe devices such as the atomic force microscope (AFM) can be used to obtain an image or other information indicative of the features of a wide range of materials with molecular and even atomic level resolution. In addition, AFMs are capable of measuring forces accurately at the piconewton to micronewton range, in a measurement mode known as a force-distance curve or force curve. As the demand for resolution has increased, requiring the measurement of decreasingly smaller forces and movements free of noise artifacts, the old generations of these devices are made obsolete. A demand for faster results, requiring decreasingly smaller cantilevers, only reinforces this obsolescence. The preferable approach is a new device that addresses the central issue of measuring small forces and movements with minimal noise, while providing an array of modules optimizing the performance of the device when using small cantilevers or when doing specialized applications such as electrical techniques, optical techniques for biology and energy research, nanoindentation and electrochemistry.
For the sake of convenience, the current description focuses on systems and techniques that may be realized in a particular embodiment of scanning probe devices, the atomic force microscope (AFM). Scanning probe devices include such instruments as AFMs, 3D molecular force probe instruments, scanning tunneling microscopes (STMs), high-resolution profilometers (including mechanical stylus profilometers), surface modification instruments, nanoindenters, chemical/biological sensing probes, instruments for electrical measurements and micro-actuated devices. The systems and techniques described herein may be realized in such other scanning probe devices.
An AFM is a device which obtains topographical information (and/or other sample characteristics) while scanning (e.g., rastering) a sharp tip on the end of a probe relative to the surface of the sample. The information and characteristics are obtained by detecting changes in the deflection or oscillation of the probe (by detecting small changes in amplitude, deflection, phase, frequency, etc.) and using feedback to return the system to a reference state. By scanning the tip relative to the sample, a “map” of the sample topography or other characteristics may be obtained.
Changes in the deflection or oscillation of the probe are typically detected by an optical lever arrangement whereby a light beam is directed onto the side of the probe opposite the tip. The beam reflected from the probe illuminates a position sensitive detector (PSD). As the deflection or oscillation of the probe changes, the position of the reflected spot on the PSD also changes, causing a change in the output from the PSD. Changes in the deflection or oscillation of the probe are typically made to trigger a change in the vertical position of the base of the probe relative to the sample (referred to herein as a change in the Z position, where Z is generally orthogonal to the XY plane defined by the sample), in order to maintain the deflection or oscillation at a constant pre-set value. It is this feedback that is typically used to generate an AFM image.
AFMs can be operated in a number of different sample characterization modes, including contact modes where the tip of the probe is in constant contact with the sample surface, and AC modes where the tip makes no contact or only intermittent contact with the surface.
Actuators are commonly used in AFMs, for example to raster the probe over the sample surface or to change the position of the base of the probe relative to the sample surface. The purpose of actuators is to provide relative movement between different parts of the AFM; for example, between the probe and the sample. For different purposes and different results, it may be useful to actuate the sample or the probe or some combination of both. Sensors are also commonly used in AFMs. They are used to detect movement, position, or other attributes of various components of the AFM, including movement created by actuators.
For the purposes of this specification, unless otherwise indicated (i) the term “actuator” refers to a broad array of devices that convert input signals into physical motion, including piezo activated flexures; piezo tubes; piezo stacks, blocks, bimorphs and unimorphs; linear motors; electrostrictive actuators; electrostatic motors; capacitive motors; voice coil actuators; and magnetostrictive actuators; and (ii) the term “sensor” or “position sensor” refers to a device that converts a physical quantity such as displacement, velocity or acceleration into one or more signals such as an electrical signal, and vice versa, including optical deflection detectors (including those referred to above as a PSD and those described in co-pending applications US Patent App. Pub. Nos. US20030209060 and US20040079142, Apparatus and Method for Isolating and Measuring Movement in Metrology Apparatus, which are hereby incorporated by reference in their entirety), capacitive sensors, inductive sensors (including eddy current sensors), differential transformers (such as described in U.S. Pat. No. 7,038,443 and co-pending applications US Patent App. Pub. Nos. US20020175677, Linear Variable Differential Transformers for High Precision Position Measurements, and US20040056653, Linear Variable Differential Transformer with Digital Electronics, which are hereby incorporated by reference in their entirety), variable reluctance, optical interferometry, strain gages, piezo sensors and magnetostrictive and electrostrictive sensors.
Some current AFMs can take images up to 100 μm2, but are typically used in the 1 μm2-10 μm2 regime. Such images typically require from four to ten minutes to acquire. Efforts are currently being made to move toward what is sometimes called “video rate” imaging. Typically those who use this term include producing images at the rate of one frame per second all the way to true video rate at the rate of 30 frames per second. Video rate imaging would enable imaging moving samples, imaging more ephemeral events and simply completing imaging on a more timely basis. One important means for moving toward video rate imaging is to decrease the mass of the probe, thereby achieving a higher resonant frequency with an equal or lower spring constant.
Conventional AFM probes are currently 50-450 μm in length with fundamental resonant frequencies (fR) of 10-500 kHz and spring constants of 0.01-200 N/m. Physical laws put lower limits on the achievable resolution and scan speed of conventional probes, given typical acceptable noise levels.
To get the best resolution measurements, one wants the tip of the probe to exert only a low force on the sample. In biology, for example, one often deals with samples that are so soft that forces above 10 pN can modify or damage the sample. This also holds true for high resolution measurements on hard samples such as inorganic crystals, since higher forces have the effect of pushing the tip into the sample, increasing the interaction area and thus lowering the resolution. For a given deflection of the probe, the force increases with the spring constant (k) of the probe. When operating in air in AC modes where the tip makes only intermittent contact with the sample surface, spring constants below 30 N/m are desirable. For general operation in fluid, very small spring constants (less than about 1.0 N/m) are desirable.
To get measurements with higher scan speeds, one wants probes with a high fR. After passing over a sample feature, the probe response is about 1/fR seconds for contact modes and Q/fR seconds for AC modes (where Q is the quality factor for the probe). This sets a fundamental limit on scanning speed: lowering the response time of the probe requires a probe with a high fR or, in the case of AC modes, either a low Q or a high fR or both.
A higher fR also means lower noise operation. The thermal noise of a probe involves fixed noise energy of order kT (where k is the Boltzmann constant and T is the temperature in Kelvin) spread over a frequency range up to approximately fR. Thus, the higher fR, the lower the noise per unit band width below fR.
Probes with a high resonant frequency and a low spring constant can be achieved by making them smaller and thinner. However, using current AFMs with probes significantly smaller than conventional ones presents difficulties. In general, optimal optical lever detection requires that the spot from the light beam directed onto the side of the probe opposite the tip should substantially fill the area along the length of the probe. Underfilling results in a loss of optical lever detection efficiency because the reflected beam diverges more than necessary. Overfilling the probe means losing light and producing unwanted interference fringes due to light reflected off the sample.
One ideal probe for video rate imaging would have a fR in the 5-10 MHz range and a force constant in the 1-40 N/m range. This implies shrinking conventional probes by an order of magnitude, to approximately 5-8 μm in length or width. Such a shrinking, taken together with the requirement that the spot substantially fill the probe, means that the spot on the probe also must be shrunk. The optical system producing the beam incident on the probe should have a numerical aperture (NA) sufficient with the wavelength of the light from the light source to form a focused spot approximately 5-8 μm in diameter in at least one direction.
The relatively large numerical aperture required to so shrink the spot results in a shallow depth of focus. This can present problems with the refocusing necessary when replacing one probe with another or when using a probe with more than one cantilever. In addition, the large opening angle of the incident beam used to achieve a high numerical aperture can require complex lens systems or an accumulation of lenses in close proximity to the probe.
SUMMARY OF THE INVENTION